Infection is a game mode in the Halo franchise that has been around since Halo 2. Since then it's gone through various changes and in the latest version of Halo, infected players now appear as the flood.
Whilst infection has been in practically every version of Halo, the game mode has not yet been added to the Master Chief Collection. However, if the latest news from 343 Industries is anything to go by then the game mode should be coming to MCC soon as well.
In the latest weekly blog post on Halo Waypoint, 343 Industries detailed their upcoming plans for the Master Chief Collection, whilst hinting at an upcoming Infection update.
"The playlist team continues to work on future updates, and it's quite likely that we'll be talking more about things like Infection in the coming weeks," 343 Industries stated in their blog post. For those unfamiliar with the infection game type in Halo - it works similarly to the standard FPS 'zombies' or 'infection' mode that has been in first person shooters for years.
The majority of players start as clean, non-infected participants and a small selection of players are randomly chosen to be the infected participants at the start of each round. The infected players must attempt to 'infect' other players by killing them. The infected have infinite lives, but non-infected players only have life before they're moved over to the infected team.
In the Halo version of Infection, the last living player gets extra bonuses such as an overshield to help them survive the oncoming infected hordes.